Walker made his choice known on a Milwaukee talk radio show whose conservative host had already endorsed Cruz. Walker, whose short-lived run for president ended in September, had previously said he would back whatever candidate is chosen as the GOP nominee. But he'd sent signals he intended to back Cruz.

Cruz said in a response on Twitter that he was "honored" to win the endorsement.

It's unclear how much Walker's backing will help Cruz in the state ahead of its April 5 primary. Walker's approval rating hasn't exceeded 40 percent in over a year.

Walker said last week that Cruz was the only candidate who had a chance at beating front-runner Donald Trump. When Walker ended his presidential bid, he called on others to join him so it would be easier to take on Trump.

Cruz pointed to Walker's career-defining push against public-sector unions in 2011, a fight that Walker won and that led to an effort to recall him from office. Walker won that election in 2012, making him the first governor to defeat such an effort, raising his national profile in advance of his failed presidential run.

Cruz said Walker's fight with the unions showed that special interests could be defeated.

"It inspired people all over the country, it inspired millions, it inspired me," Cruz said. "It is that spirit, that fight, that's at the heart of what our campaign is all about."

When he was running for president, Walker argued that the next president should be a governor with executive experience. But Ohio Gov. John Kasich, while campaigning last week in Wisconsin, said he had no idea whom Walker was going to back.